Swimming pools

Why it is not a tank like any other

In a basement, water pushes from the outside and intermittently. In a swimming pool it pushes from the inside, always, carrying chlorine or thermal salts with it. On top of that comes the emptying-and-filling cycle, which puts the coating under tension, and in outdoor pools the thermal swing between water and air.

The coating must therefore be flexible, able to adhere to the substrate even under positive pressure from inside, resistant to chlorinated water, and compatible with the adhesive used for the tile or mosaic that will cover it.

The correct build-up

It starts with the substrate: the pool's concrete must be sound, clean, with fillets already formed at the corners between walls and floor. A consolidating primer is applied on absorbent or poorly cohesive substrates.

Next comes the actual waterproofing: a flexible two-component cementitious slurry, with crack-bridging capability and declared resistance to chlorinated water, applied in two crossed coats. This is the system specifications refer to with the code CM O1 P under EN 14891. Where fast turnaround is needed, a fast-reactive hybrid waterproofing is used, ready for recoating within a few hours.

Tile or mosaic is then laid on top with adhesive and grout suited to permanent immersion.

The points that make a pool leak

It is never the flat wall. It is the corners between floor and walls, the expansion joints, the inlet and return fittings, the skimmer, the main drain, underwater lights and service penetrations.

Each must be solved with its dedicated component: elastic sealing tapes embedded in the slurry coats, pre-formed internal and external corners, sleeves for penetrations, elastic polyurethane sealants on joints. A system is judged by the completeness of these accessories, not by the product spread over the flat surface.

A structurally waterproof tank

On large pools, sports pools and thermal facilities it is worth adding a second line of defence: making the pool's concrete waterproof with a crystalline admixture dosed at the batching plant, and treating construction joints with waterstops. The internal coating stays, but it is no longer the only barrier — and on a facility that cannot stay out of service, that redundancy is worth its cost.

For thermal pools, compatibility with the specific water composition must be checked, as it can be far more aggressive than chlorinated water.

Testing and filling

The watertightness test is carried out once the waterproof coating is complete and before laying the finish, filling the pool and measuring the drop in level over time, net of evaporation. The first definitive filling should be done gradually so as not to stress the structure, respecting the curing times of adhesives and grout.